In the NTSC television signal system, which is the system presently in use in the United States and in many other countries, the transmission specifications and format were established in order to maximize the amount of red (R), green (G) and blue (B) primary color information which can be transmitted in the television signal. Thus, the NTSC specifications and format provide wide chromaticity ranges in order to insure transmission of full color content for the television image.
Television receivers known in the prior art reproduce the color image contained in the television signal by adding and mixing the red, green, and blue primary color information extracted from the television signal. However, the chromaticity ranges of a typical television receiver is narrower than the chromaticity ranges present in the received television signal. Thus, full and faithful reproduction of the color picture information contained in the television signal is not possible in television receivers known in the prior art.
FIG. 1 is a CIE (Commission International de Enluminure) chromaticity diagram illustrating the red, green and blue primary color chromaticity values for the NTSC system, the phosphor in a cathode ray tube (CRT) used in the typical television receiver and the color filter in a liquid crystal display device (LCD), also now commonly used in small portable television receivers and large projection-type television displays. The NTSC chromaticity values are represented by the dashed line, the CRT chromaticity values are represented by the dot-dash line and the LCD values are represented by the solid line. The chromaticity values charted in FIG. 1 are set forth below in table form:
______________________________________ NTSC CRT LCD X Y X Y X Y ______________________________________ R: 0.670 0.330 0.657 0.338 0.613 0.334 G: 0.210 0.710 0.297 0.609 0.283 0.627 B: 0.140 0.080 0.148 0.054 0.140 0.079 ______________________________________
The outer bold line in FIG. 1, and associated numbers at the indicated points on the line, illustrate the spectrum locus and wave-lengths (nm). In the NTSC system here, C light having a color temperature of 6774 K is used as a reference white light W with the following chromaticity: EQU W:X=0.3101, Y=0.3163
In the NTSC system, if the chromaticity values for the three primary colors for the display (CRT or LCD) in the receiver are set to the same as the NTSC values, the color picture information in the transmitted television signal can be faithfully reproduced on the receiver display by setting the reference white light to the chromaticity of C light. However, a color television receiver must also be capable of receiving and displaying black and white television signals. Therefore, in order to achieve the requisite compatibility with black and white television broadcasts, a white color light with higher chromaticity values than C light must be used as the reference.
In addition, the chromaticity values for the primary colors for a television receiver depends on the phosphor chromaticities of the CRT or LCD which, as described above and shown in FIG. 1, are different than the chromaticity values for the primary colors for the NTSC system. Therefore, if corrections are not made for the differences between the NTSC and receiver chromaticity values, color errors will result and the color picture information contained in the television signal will not be faithfully reproduced on the display in the receiver.
Moreover, and as also discussed above, the chromaticity ranges of the CRT and LCD are narrower than the NTSC chromaticity ranges. Thus, the color reproduction range of the television receiver is not broad enough to reproduce all of the color information contained in the incoming television signal. Therefore, even when the appropriate color corrections are made with respect to differences in the NTSC and display chromaticity values, the receiver is still not capable of faithful reproduction of every color received in the incoming television signal.
The types of color reproduction errors discussed above are also referenced on page 959 in the book entitled "The New Color Science Handbook" which was published in Japan.
FIG. 2 is a UV chromaticity diagram showing the color reproduction errors described above and referenced in the above mentioned handbook. The solid line triangle in FIG. 2 indicates the chromaticity range for the primary colors of the NTSC system on the transmission side of the television signal. The dashed line triangle illustrates the chromaticity range of the primary colors of the CRT phosphor at the receiver. Note that while the NTSC and CRT ranges greatly overlap, the NTSC range is slightly larger and each of its values are shifted from the corresponding values in the CRT range. The graph in FIG. 2 is obtained by calculating the color reproduction error based on the differences between the NTSC and CRT chromaticity ranges. An example of some of the NTSC chromaticity values are shown by the dots in FIG. 2. In order to correct color reproduction errors in the display, these values must be shifted in order to bring them into a corresponding point in the CRT chromaticity range as indicated by the arrows in FIG. 2.
In CRT television receivers and color cameras, linear matrix circuits are used to correct color reproduction errors. FIG. 3 is a block diagram showing a linear matrix circuit which corrects such color reproduction errors. The G (green), B (blue) and R (red) primary color signals are supplied to input terminals 1, 2 and 3, respectively, and to gamma cancellation circuits 4, 5 and 6 so that transmission gamma is cancelled by removing the gamma correction which was added to the signal on the image transmission side.
The output signals from gamma cancellation circuits 4, 5 and 6 are supplied to matrix circuit 7 which is formed of coefficient circuits 8 to 16 and adders 17 to 19. Matrix circuit 7 multiplies the supplied R, G and B signals by respective correction matrix coefficients and then adds them together. Coefficient circuits 8 to 16 can be formed of simple circuits using resisters as is known in the art. Gamma addition circuits 20, 21 and 22 add the transmission gamma to the respective corrected R, G and B color signals and outputs them via terminals 23, 24 and 25 as primary color signals R.sup.1, G.sup.1, and B.sup.1. In this way, color reproduction errors are removed and approximately the same colors as in the transmitted image can be reproduced at the receiver. However, the correction is possible only for colors in the region in which the solid line triangle and the dashed line triangle in FIG. 2 overlap each other.
Recently, there have been test broadcasts of a high definition television (HDTV) signal format which is capable of displaying high quality television images. Taking into account the popularity of the existing NTSC television system, and the large installed base of such equipment, television receivers which are capable of receiving images in both the NTSC and HDTV system formats are being made commercially available. However, the transmission specifications of an HDTV system are different from those of an NTSC system. In the HDTV transmission specification and format, the R, G and B primary color chromaticities are as follows:
______________________________________ R: X = 0.640, Y = 0.330 G: X = 0.300, Y = 0.600 B: X = 0.150, Y = 0.060 ______________________________________
Also in the HDTV specification, D.sub.65 light is used as a reference white color and its chromaticity W is as follows: EQU W:X=0.3127, Y=0.3290
FIG. 4 is a CIE chromaticity diagram showing the NTSC specification values and the HDTV specification vales of the respective R, G and B primary color chromaticities. As FIG. 4 indicates, the chromaticity value for G in the NTSC system is greatly different than the chromaticity value for G in the HDTV system. Note, however, that the chromaticity value for G in the HDTV system is comparatively close to the chromaticity value for G in CRT and LCD displays (see FIG. 1). Therefore, in a television receiver which uses a CRT or LCD, there is comparatively little color reproduction error when receiving and displaying a HDTV broadcast. There is, however, a likelihood of large color reproduction errors when receiving an NTSC broadcast as is apparent from FIG. 1.
When correcting color reproduction errors for one system in a television receiver which is capable of receiving multiple system formats (such NTSC and HDTV) in which the three primary color chromaticity values differ from one system to the other, color reproduction errors in the other systems cannot be corrected.
Television programs produced by the HDTV system and commercial feature films are sometimes broadcasted using the NTSC system. Feature films usually are produced in accordance with specifications established by the Society of Motion Picture and Television Engineers (SMPTE). The HDTV system also is set to conform to SMPTE specifications.
FIG. 5 is a CIE chromaticity diagram showing the transmission primary color chromaticity values for both the SMPTE and NTSC systems. As is apparent from FIGS. 4 and 5, the transmission primary color chromaticity values for the HDTV system approximately coincides with those of the SMPTE system. Also, the PAL and SECAM television signal transmission systems are nearly standardized to the EBU specifications. The transmission primary color chromaticity values for these systems also approximately coincide with those of the HDTV system
There is presently known in the art an HDTV to NTSC system converter which enables reception of HDTV broadcasts on a standard NTSC system television receiver. With such a converter, or other equipment having similar functions as that of the converter, it is possible to also convert other signal formats, such as SMPTE, PAL and SECAM to NTSC for display on a standard NTSC receiver. However, the transmission gamma of the NTSC system differs from the transmission gamma of any one of the other systems.
FIG. 6 is a graph illustrating transmission gamma. The solid line shows the gamma curve of the SMPTE system and the dashed line shows the gamma curve of the NTSC system (.gamma.=0.45). As shown in FIG. 6, the black side values in particular differ between the NTSC and the SMPTE systems. Due to the difference between the gamma curves, the rendering of the black gradations sometimes is unnatural depending on the signal source of the broadcast program. For instance, even within the same broadcast program, the gradation rendering differs according to the difference in the signal source, namely in accordance with the specification under which the program was produced.
In addition to television receivers which use a CRT or liquid crystal display, recently projection-type color television receivers have been developed in order to provide large-screen viewing. Projection-type television receivers uses a plurality of liquid crystal panels capable of modulating light colors. Such a receiver is referred to at pages 415 to 418 in "SID 91 DIGEST."